Accumulators as a rule consist of a plurality of accumulator cells, which together are connected to packs or modules, from any desired type of accumulator, such as, for example, NiCd, NiMh and Li-ion, and are therefore also designated as an accumulator pack or an accumulator module. In particular accumulators with Li-ion cells are widely used on the basis of their high specific capacity as well as their high specific energy content. Li-ion cells, as they, for example, are used in electrical tools, are typically configured as round cells with a rigid metal cylinder, which is sealed with a cover which forms a terminal of the cell. Because an excess of pressure can arise when gas forms during the operation of such Li-ion round cells on account of electrochemical processes inside the cell, for example in the case of an electrical overload, the Li-ion round cells can be equipped with suitable safety devices. Provision, for example, can be made for a safety valve in the cover of said cells, which opens to blow off vaporizing electrolyte when excess pressure occurs, and/or a bursting disc, which is ruptured when excess pressure occurs and an associated electrical connection in the round cell is interrupted.
The Li-ion round cells have an increase in their overall mass, which does not contribute to providing energy, on account of the mass of their metal cylinder; and their round construction leads to dead construction volume in corresponding accumulator packs or modules. Because of these two factors, it is known from the technical field to revert back to so-called “lithium polymer” cells (LiPo cells) in order to avoid this increase in both mass and dead construction volume. These typically have electrode assemblies, which consist of platelet-like electrodes stacked on top of each other. Said electrodes are respectively separated from each other by suitable separators and are disposed in an elastic cell sheath. Such LiPo cells are as a rule prismatic and can consequently be disposed in the housing of an associated accumulator pack or module in order to save space.
The disadvantage in the technical field is that in contrast to a rigid metal cylinder, the elastic cell sheath expands when a change in volume of the accumulator cell occurs, for example when gas forms as described above, on account of the emergence of excess pressure. This expansion of the cell sheath takes place however without a controllable pressure build-up, which is required to release one of the safety devices described above, and can lead to the damage or destruction of the cell sheath and consequently of the cell, respectively of an associated accumulator pack or module.